1. Field of the Invention
The present invention relates to a method for forming a semiconductor. Particularly, the present invention relates to a method for forming a semiconductor film, which has an excellent crystalline property and contains less impurity in a crystalline film. The semiconductor film prepared in accordance with the method of the present invention can be used in such semiconductor device as a high quality thin film transistor having a high field effect mobility.
2. Description of the Related Art
So far, a method for crystallization of a non-crystalline silicon hydride film formed by a plasma CVD or a heat CVD method, which contains a lot of hydrogen, and a lot of bonding hands of silicon which are neutralized with hydrogen (it can be described as a-Si:H), by irradiating such laser light as CW (Continuous-wave) laser and Excimer laser has been well known.
This method, however, poses a problem that a non-crystalline silicon film as a starting film contains a lot of hydrogen, much of which spouts from the film when the film is subjected to a laser irradiation, and then a film quality deteriorates remarkably. To overcome this problem, there have been used chiefly the following three methods.
(A) At first, by irradiating a low energy density (less than threshold value energy for crystallization) laser light to a non-crystalline silicon hydride film as a sample, hydrogen in the starting film as a sample will be drawn out. And then, by irradiating a high energy density laser light to the sample, the sample will be crystallized. This is called as a multi-stage irradiation method.
(B) By forming a non-crystalline silicon hydride film, at 400xc2x0 C. or more of a substrate temperature, a hydrogen content in the starting film will be decreased. Thus, a film quality deterioration by a laser irradiation will be prevented.
(C) By effecting a heat treatment of a non-crystalline silicon hydride film, in an inactive atmosphere, hydrogen in the film will be removed.
The following problems, however, have been posed, in order to obtain a high quality crystal film (in general, polycrystal silicon film) by the above mentioned methods.
(1) The method of (A) has a problem in a practical use. Namely, it is bad in an efficiency of hydrogen drawing out, difficult to control a laser energy output, and required to increase the number of laser irradiation times. Since almost all the laser light energy is absorbed in the film surface, it is difficult to draw out hydrogen, in case of a thick film.
(2) The method of (B) has a problem that a crystallization is obstructed in a later process of crystallization by a laser light. This is caused by that an impurity content in the film is inclined to increase, when a substrate temperature becomes high, and a silicon cluster (microcrystal part of silicon) is formed, if a non-crystalline silicon hydride film is formed at a high temperature.
(3) The method of (c) has a problem that an electric property (carrier mobility etc.) of the crystal film deteriorates, caused by such impurity as oxygen. This is derived from the reason that since hydrogen is eliminated by heating, a dangling bond (unpaired bonding hand) of silicon is formed, and the dangling bond combines easily with oxygen etc. then, the oxygen intrudes easily into the film from its surface to 10 to 20 nm in depth, and diffuses into the film deeply by a high temperature diffusion.
Accordingly, it is an object of the present invention to solve the above mentioned problems. The object can be accomplished by adopting a method for forming a semiconductor in accordance with the present invention. Such a method in accordance with the present invention is described below (a) to (c).
(a) A non-crystalline (amorphous or microcrystal) silicon hydride film having a Si-H bond of high density is formed on a substrate, at a low temperature by a gas phase chemical reaction (chemical vapor deposition). As a method of the gas phase chemical reaction, such conventional method as a plasma CVD (chemical vapor deposition) method, a heat CVD (thermal chemical vapor deposition) method, and a light CVD (photo chemical vapor deposition) method can be employed. At this time, it is needed to form a non-crystalline (amorphous or microcrystal) silicon hydride film at 350xc2x0 C. or lower of a substrate temperature, which is said to be an eliminating temperature of hydrogen from the non-crystalline (amorphous or microcrystal) silicon hydride. This is to increase the number of bond between silicon and hydrogen (Si-H bonding), by making hydrogen contain in a film as much as possible, in a film forming process. It is preferable to form a film at a substrate temperature as low as possible, in order to maximize Si-H bonding. However, in an actual use, it is possible to achieve the maximizing object of Si-H bonding, by forming a film at 100 to 200xc2x0 C. of the substrate temperature.
Also, it is possible to prevent a silicon cluster (microcrystal of silicon) from being formed in a film, by effecting a film forming at a low temperature, and to expect a more uniform crystallization, at a later crystallizing process.
(b) Hydrogen in a non-crystalline (amorphous or microcrystal) silicon film is expelled from the silicon film and a high density dangling bond is formed in silicon, by effecting a heat treatment (thermal annealing) of the non-crystalline (amorphous or microcrystal) silicon film, which was formed in the above mentioned (a) process, in a vacuum or inactive gas atmosphere. The heat treatment in a vacuum or inactive gas atmosphere comes from that a combination (bonding) of the dangling bond of silicon and such impurity as oxygen is prevented to the utmost. It is important for the heating temperature that the substrate temperature be 350xc2x0 C. or higher and also 500xc2x0 C. or lower. This is based on that the eliminating temperature of hydrogen from a non-crystalline (amorphous or microcrystal) silicon hydride is about 350xc2x0 C., and the crystallization starting temperature of a non-crystalline (amorphous or microcrystal) silicon is about 500xc2x0 C. Also, since there is a case where the crystallization starts at 450xc2x0 C., when an impurity concentration, especially oxygen in the film is low, it is preferable that this heating process to draw out hydrogen is carried out at 400xc2x0 C. extent in an actual use. It is suitable that the time for the heating process is in the degree of 30 min. to 6 hrs. The inactive gas atmosphere comprises a material selected from the group consisting of N2, Ar, H2, He and a mixture thereof. In case of the inactive gas atmosphere, the heated amorphous or microcrystal film is irradiated with the laser light through a cap layer provided on the heated amorphous or microcrystal film.
Pressure of the inactive gas atmosphere is e.g. 0.5 atm. to 1.5 atm.
This heating process is to be carried out, so as to produce a lot of dangling bond, by releasing hydrogen made be contained in a non-crystalline (amorphous or microcrystal) silicon in the above (a) process. Moreover, this producing a lot of dangling bond is to make the crystallization easy, in a later crystallizing process by a laser irradiation or heating. So that it is not preferable that the non-crystalline (amorphous or microcrystal) silicon film crystallizes in this heat treatment step. The reason why the crystallization (including a cluster state in the very small region) in this heat treatment step should be avoided is based on the fact that once crystallized film will not give a good quality in an electric performance, on the contrary, it will deteriorate, even if an energy for the crystallization (e.g. irradiation energy of laser) is added.
Also, afterward, there is a crystallizing process for non-crystalline (amorphous or microcrystal) silicon film by a laser irradiation or a heating. But, it is very effective in the improvement of film crystalline property in a crystallizing process by the later laser irradiation or the heating, to maintain an atmosphere in a vacuum or inactive state and to avoid a combination (bonding) of impurity with the dangling bond of film to the utmost, up to this crystallizing process.
(c) A non-crystalline (amorphous or microcrystal) silicon film is crystallized by a laser irradiation or a heating with the vacuum or inactive gas atmosphere maintained. In this process, it is extremely important not to break the vacuum state or inactive gas atmosphere in succession with the above (b) process. This results from the reason that silicon in the non-crystalline silicon film is apt to react with impurity exceedingly, as a dangling bond has been formed with high density in the non-crystalline (amorphous or microcrystal) silicon film during the heating (thermal annealing) process (b). The non-crystalline (amorphous or microcrystal)silicon film is maintained in a vacuum or inactive gas atmosphere from the process (b) to the process (c). The non-crystalline (amorphous or microcrystal) silicon film is irradiated with a laser light or heated at a temperature of 500xc2x0 C. or higher in the process (c). The process (c) is carried out after the process (b).
Also, it is effective in the improvement of crystalline property to apply a laser irradiation to a substrate, heating it at about 300xc2x0 C. to 500xc2x0 C., so as to lower a cooling speed of the substrate temperature, at this occasion. In case of the crystallizing by only heating, it can be effected by heating at a temperature of 450xc2x0 C. or higher, typically 450xc2x0 C. to 800xc2x0 C., but in general, it is conducted by heating at 600xc2x0 C. degree for 1 hr. to 96 hrs. considering a heat-resistant temperature of a glass substrate.
In the claim of the present invention, it is defined that a non-crystalline (amorphous or microcrystal) silicon starts on crystallization at 500xc2x0 C. or higher. But, it starts on it at the degree of 450xc2x0 C., if an oxygen concentration in a film is very low. Therefore, it is defined as such.
In the present invention, it is important to form a non-crystalline (amorphous or microcrystal) silicon hydride film which has high density bonds between silicon and hydrogen, as a starting film, and to obtain a non-crystalline (amorphous or microcrystal) silicon film having a high density dangling bond, drawing out hydrogen from the starting film, through the heat annealing process to accelerate a dehydrogenation from the film. The employment of such film is due to the fact that the film has a tendency to crystallize, as a non-crystalline silicon film having a high density dangling bond of silicon is active in a lattice vibration of an atom level, and is in a very unstable condition thermally. And then, a polycrystal silicon semiconductor film is obtained by crystallizing the non-crystalline (amorphous or microcrystal) silicon film, which has been apt to crystallize, after the dangling bond was formed with high density, by a laser irradiation and a heating for the crystallization.
Also, it is important to maintain the non-crystalline (amorphous or microcrystal) silicon semiconductor film in an atmosphere isolated from the open air, in the above mentioned process, i.e. from the film forming (film deposition) process (a) to the crystallization process (c) through the heating process (b). This is to prevent the dangling bond from combining with oxygen etc. to the utmost. And in order to attain this object, an equipment having a chamber, which is provided with a high vacuum exhaust system, a quartz window for laser irradiation, a heating apparatus for heating process and the like, is needed. In an industrial scale, a multi-chamber type equipment provided with the above mentioned apparatus or device is useful.